3 Answers
3

For the tl;dr crowd who only care about the usage I will give you a quick break downs:

Group your Mutalisks

Move them out of harms way and press "stop"

Wait for them to spread out

Move (not attack move) near by the Thor

No two Muta should be hit by the splash

Once the Mutalisks stop on their own you can Focus Fire the Thor

Remember: after the Thor dies you will need to move (not attack move) to the new target or risk bunching up.

I'm going to quote myself on this one:

Right now, the Magic Box is one of the most interesting things happening in Starcraft 2 game play. At its core is the idea that “Hard Counters” don’t really exist in Starcraft 2 and that, rather, how you use your units is more important than what they are. Those people familiar with using Speedlings against Hellions or Tanks against Marauders have already encountered this strange Phenomenon.

To help you understand this, it’s probably best if I start by explaining what a “Hard Counter” is. The concept of a counter unit is very much based in the children’s game Rock-Paper-Scissors. For those unaware of the game, children simultaneously choose one of the three titular elements, each of which wins against another element while losing against the other differing element (same elements are considered draws). A first glance at Starcraft seems to indicate a similar trend: Hellions do extra damage to light units (which Zerglings are) and can be upgraded even further in this capacity; by comparison Zerglings do not do extra damage to Hellions and, further more, clump up causing them to take additional damage from the Hellions’ AoE . This would imply to the layperson that an army of Zerglings will lose to an army of Hellions. Strangely enough, you’ll see many Zerg players actually employ Speedlings (speed upgraded Zerglings) in defense against Hellions. While this may seem counter intuitive, if the Speedlings can surround the Hellion (thus preventing escape) they are very effective at killing it.

As Day9 might say, “[Hard Counters] deeply bother me.”

The Magic Box is a technique which gives Mutalisks the advantage against Thors. For a long time people considered Thors to be the “Hard Counter” to Mutalisks, and it’s not hard to see why. A Thor costs 300 Minerals and 200 Gas, has a DPS of 8 (or 16 vs Light, which Mutalisks are) and 400 Hit Points. Since gas is often a limiting factor in building costs, conventional philosophy dictates that you could build 2 Mutalisks for the same price (100 Gas each), each doing 5.92 dps (or 8.55 spread across three targets), and having 120 Hit Points. The armchair math is thus, 400 HP @ 8.55*2 dps vs 240 HP @ 16 dps. It’s not hard to see which will die first. What’s even worse, for the Zerg player, Thors do splash damage, and Mutalisks have a tendency to bunch up (as air units often do). As a result, a sufficient force of Thors could wipe out a much larger force of Mutalisks.

This sort of armchair theory crafting was debunked very early on. A good example of this can be found in Day9 Daily #125. Sen demonstrates that Mutalisks build much faster and, with a little Micro, can avoid most of the damage when there are few Mutalisks. The problem with this approach is that in larger pushes, mass Mutalisk was still very susceptible to Thor heavy armies, as they require a lot of micro to reduce this damage (the production difference also is minimized in larger pushes). That was until IdrA vs Tarson.

For a moment I’d like you to put yourself in Gregory “IdrA” Field’s shoes:

After the early bracket play, you have just emerged the Leader of your group only to struggle (3-2) in the Quarter Finals of the IEM against a relatively unknown Terran, SarenS. A long time proponent of “TvZ is imbalanced,” you’re faced again in the semi-finals against another Terran player, Tarson. Once again, you’re forced into your least favorite match up, where you constantly feel you have to outplay your opponent by a significant margin just to beat an “inferior” player. Imagine how that must feel for you, once again to be face to face with an unfair system.

Right before the match, however, one of your friends and practice partners comes up to you saying, “I just had this crazy idea. Remember how if you move, not attack move, with Mutalisks, they stay spread out? And it always makes them harder to micro with? What if, and just thinking here, in Muta vs Thor, you don’t micro them into clusters?” You give him a funny look, why would you micro into clusters against Thors? “No, I mean you just move instead of attack moving, so your Muta are already spread out.” Seeing your apprehension, he adds on, “I tried it last night, there is no splash.” The conversation is cut short as you’re rushed on stage for your match, the words “no splash” ringing somewhere in the back of your head.

Somewhere during game 3, you find yourself in just such a situation. Tarson is pushing out with a heavy Mech force backed up by a number of Thors, and you see yourself with a Muta heavy army. All of sudden those words echoing in the back of your head ring true. Spreading out your units you use, what will be later known as, the magic box. The Thors crumple under the weight of your Mass Mutalisks with only a single one dying, and every Zerg player watching at home has the exact same reaction: “OH EM GEE!”
It’s a story with a happy ending (if you’re a Zerg).

Idra would go on to win 3-1 vs Tarson before losing to an unexpected build in the finals (MarroW’s 5 Rax Mass Reapers), but those are just details. Within hours, forums would be flooded with questions and experiments on The Magic Box. There are a couple good videos demonstrating its use, but I find the following to be the best:

Depending on how close together a selection of units are, they may or may not stay in formation when you order them to move. When the units are spread out too much, they will converge into one location instead of moving in formation.

The magic box refers to the maximum distance the units can be from each other for this behavior to occur. Magic box technique or magic boxing refers to taking advantage of this quirk when moving your units around, say to avoid splash damage from Thors when attacking them with Mutalisks.

This is slightly misleading: the units do not converge based on spreading but rather the distance of the waypoint from the center of mass. A sufficiently far waypoint will not cause convergence
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tzenesOct 21 '10 at 19:26